1. Field of the Invention
The present invention relates to fibre reinforced cement products and methods for modifying the properties of those products for particular purposes.
2. Description of the Related Art
Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
Generally, fibre reinforced composites may be divided into three groups based on their density.
High density reinforced composites have a density range above 1.6 to about 1.9 g/cm3. These composites may be formed in a conventional fashion involving matting of fibres, catchment of slurried fines and dewatering, eg the Hatschek process followed by compression of up to 30 MPa pressure to the desired thickness.
Such high density FRC materials have high structural strength, high stiffness and a smooth finish. One particularly desired advantage of high density products is their ability to resist moisture ingress thereby retain as-manufactured physical or chemical properties in service.
Unfortunately, however, many high density FRC products do not have good handlabilty, nailability and can be difficult to score and snap. There is also a high capital and maintenance cost involved in the production technique.
Medium density FRC products with a density from about 1.2 to 1.6 g/cm3 overcome some of the difficulties mentioned above. Even though they are normally formed in a conventional fashion, eg Hatschek process, they can be produced for relatively low cost compared with high density FRC products, have improved workability, ie handleability, score-snap, nailability and provide adequate structural strength and stiffness for most applications. Further, they generally have acceptable in service performance.
Conventional medium density FRC products, however, may not generally have the same level of resistance to moisture ingress and ability to maintain in-service performance as high density products. Further, they may not provide the flat smooth surface produced on high density products without additional coating and/or sanding.
Low density fibre reinforced composites with a density of around 0.8 to 1.1 g/cm3 are also formed in a conventional fashion, e.g. Hatschek and normally incorporate a density modifier.
These low density products have excellent workability, i.e. handleability, score and snap and nailability due to their low density. They provide acceptable in-service performance and have adequate stiffness.
Such low density products, however, generally have lower structural strength and stiffness. Ability to maintain physical and chemical properties in service is generally lower and once again, surface flatness could be improved. Due to the specialised formulation of most low density fibre reinforced composites, they are produced at a relative high cost.
Accordingly, it will be appreciated by persons skilled in the art that it would be useful to be able to modify the properties of a medium or low density product such that they retain their advantageous properties, eg workability relatively low cost etc, but improve other properties, eg ability to maintain as-manufactured properties, resistance to moisture ingress, structural strength and stiffness and surface flatness.
Performance in extreme climactic conditions is a particularly difficult area. For example, in many geographical locations, the FRC product may be subject to many freeze/thaw cycles during its life. Some conventional medium density FRC material produced may suffer from delamination, softening or chipping when exposed to freeze/thaw cycles.
Loss of as-manufactured physical or chemical properties can also be triggered by internal factors such as imperfections relating to the material heterogeneity, eg air pockets, segregation of constituents. In the case of fibre reinforced composite materials, imperfections such as poor bond at the matrix-fibre interface and fibre clumping may render such materials more susceptible to loss of durability.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.